A testimony from a fiendish undercutter!
If I were to undercut by a small value say 1 gil. I will come back in 20 minutes and see that none of my items have sold and now the other items are listed 1 or 100 gil below my items.
I aim to undercut by a sufficient enough margin that other sellers are hesitant about undercutting me.
Let's say 10 of a given item are going to sell, and there are 10 items currently up for sale at say 5k. Then I put 10 of the item up for 4900. Throughout the course of the day let's say I sold 3 items before I was undercut. I have made 14,700. Now let's say I put 10 items up for 3k. Other sellers are hesitant to lower their prices to this value long enough so that all 10 of my items sell before I am undercut. In this situation I have made 30k.
I will then make 10 of a completely different item and repeat the process, as to prevent supply overwhelming demand.